To change the size of your brush, slide your finger left and right on your hand UI.
In the menu, you can find various brushes and special effects, fire for example.
At the top of the menu, you can switch to the color wheel tab and on the sides, you can find a palette with your favorite colors.
At the bottom you can find your operations: you can draw freehand, with shapes such as lines, circles, and spheres.
You can also erase any of your brush strokes.